No "guy" owns all the diamonds minds.
De Beers has two shareholders: Anglo American (85%) and the Government of the Republic of Botswana (15%).
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Anglo American plc is a multinational mining company headquartered in London, United Kingdom. It is the world's largest producer of platinum, with around 40% of world output, and a major producer of diamonds, copper, nickel, iron ore and metallurgical and thermal coal.[3][4] It has operations in Africa, Asia, Australasia, Europe, North America and South America.[5]
Anglo American has a primary listing on the London Stock Exchange and is a constituent of the FTSE 100 Index. It had a market capitalisation of approximately £31.2 billion as of 23 December 2011, the 15th-largest company of any company with a primary listing on the London Stock Exchange.[6] It has a secondary listing on the Johannesburg Stock Exchange.[4]
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The Oppenheimer family divested their remaining shares of De Beers, whereby Anglo-American acquired an additional 40% stake for $5.1 billion, which increased Anglo-American stake to 85%. This came at a time of increased labour strikes and international attention to Oppenheimer's involvement in blood and conflict diamonds.[25][26] The Oppenheimer family remains the largest shareholder of Anglo-American.
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Nicholas "Nicky" F. Oppenheimer (born 8 June 1945) is a South African businessman and philanthropist. He was formerly the Chairman of De Beers diamond mining company and of its subsidiary, the Diamond Trading Company, and former Deputy Chairman of the Anglo American Corporation.
TL;DR: There is a "guy" who owns all the diamond mines, and it is a 69 year old white man named Nicky Oppenheimer.